“Snubbing” relates to the insertion of piping and tools into, or their extraction from, wells being serviced/drilled for oil and natural gas under pressure. It is conventional to employ one or more blow-out preventers BOPs in conjunction with such snubbers. The BOPs are closed into pressurized engagement with the piping in order to prevent leakage of gas under pressure from the well bore.
Conventionally, a snubbing unit is situated on top of the primary service/drilling rig BOPs. One or more sets of stripping rams are used to strip pipe sections out of and into the pressurized well.
Snubbing is done when pipe sections are pulled out/pushed into a well under pressure. Normally this pressure is from natural gas. Stripping rams and stripping annulars, components of a snubbing unit, are designed to have pipe slide through them. Pipe connectors (i.e. collars) cannot be slid through the stripping rams because of their size. When a collar is stripped from a higher pressure well, it must be “staged” out. To stage a collar out from a well, a combination of stripping annulars and stripping rams are used in accordance with the following sequential steps:                1. The pipe is hoisted by the service rig until the collar is below a stripping annular. The collar is at the stripping annular when the service rig weight indicator starts to increase in weight.        2. The adjacent stripping rams are closed.        3. The pressure that is trapped between the stripping rams and the stripping annular is then bled off to atmospheric pressure.        4. The stripping annular is then opened.        5. The service rig then hoists the collar above the stripping annular.        6. The stripping annular is then closed.        7. The pressure is then equalized from below the stripping rams to between the stripping annular and the stripping rams.        8. The stripping rams are then opened.        9. Then the pipe is hoisted to the next collar and the process starts again.        
A difficulty currently exists in that, at the eighth step, the operator may forget to open a stripping ram. When this happens, a collar can be accidentally pulled into the closed stripping ram and dislodged, causing pipe sections to separate. This creates a number of possible dangers, including exposing workers to injuries from flammable gas or an explosion. For example, an incident occurred in November 2004 on a gas well in Alberta where there was a service rig and a snubbing unit on the well. The pipe was being hoisted out of the well and the workers forgot to open a stripping ram. The pipe sections separated and there was an explosion. That explosion killed the service derrick hand, burned two workers and damaged the service rig, snubbing unit and BOPs.
Thus it is an object of the present invention to provide a system, adapted for snubbing units with BOPs in conjunction with a service or a drilling rig that prevents the pipe from moving when the stripping rams are closed.